Recently, there has been demand for the development of an ultra-thick steel sheet having high strength properties in consideration of the design requirements of structures to be used in the shipping, maritime, architectural, and civil engineering fields domestically and internationally. In a case in which high-strength steel is included in the design of a structure, economic benefits may be obtained due to reductions in the weight of structures while processing and welding operations may be easily undertaken using a steel sheet having a relatively reduced thickness.
However, as in the case of ultra-high strength steel, during welding operations, the microstructure in a weld heat-affected zone (HAZ) includes low-temperature transformation phase having high strength, there is a limitation in which the weld HAZ properties, in detail, toughness, is significantly reduced. For this reason, it is significant to secure the toughness in a welding zone in terms of characteristics of a structural material, but it may be technologically very difficult to simultaneously secure the properties of a base material and a welding zone in the case of ultra-high strength steel having a tensile strength of 800 MPa or greater.
In the meantime, in the case of the related art high-strength steel having a tensile strength of 600 MPa or greater, the microstructure in a weld HAZ is fine using a TiN precipitate to secure the welding zone properties (Patent Document 1), or the generation of intergranula ferrite suppressing the generation of upper bainite in the weld HAZ is promoted using an oxide metallurgy technology to improve the toughness in the weld HAZ (Patent Document 2).
However, in the case that ultra-high strength steel having a tensile strength of 800 MPa or greater is welded, the weld HAZ generally consists of a structure such as martensite having significantly low toughness, rather than an acicular ferrite structure or a bainite structure. In addition, in the case that the martensite structure is formed, the effect of grain fining caused by the creation of TiN precipitates has a limitation in securing the toughness of the weld HAZ. Furthermore, in the case of oxide metallurgy technology, the possibility of the application thereof is relatively low, due to questions about the effectiveness thereof.
Patent Document 1: Korean Patent Laid-Open Publication No. 2009-0069818
Patent Document 2: Korean Patent Laid-Open Publication No. 2002-0091844